1. Field of the Invention
The present invention is related to an uninterruptible power system (UPS), and more particularly to a power-saving line interactive UPS.
2. Description of the Related Art
With reference to FIG. 4, a conventional line interactive UPS has a power switch set (50), a low-frequency transformer (51), a full-bridge switching circuit (52), a main controller (53), a charge and discharge mode controller (54) and a rechargeable battery (55).
The power switch set (50) is serially connected between an AC power supply and a load and determines if the AC power supply supplies power to the load.
The low-frequency transformer (51) has two windings (511a, 511b). One of the windings (511a) is connected to the load and connected to the AC power supply through the power switch set (50).
The full-bridge switching circuit (52) has two half-bridge switching circuits (521, 522). Serially connected nodes of the two half-bridge switching circuits (521, 522) are connected to the other winding (511b) of the low-frequency transformer (51).
The main controller (53) is connected to the power switch set (50).
The charge and discharge mode controller (54) is connected to the main controller (53) and the full-bridge switching circuit (52), and activates the full-bridge switching circuit (52) to enter a charging or discharging mode in accordance with the charge command or a discharge command of the main controller (53).
The rechargeable battery (55) is connected to the full-bridge switching circuit (52) and performs charging or discharging through the full-bridge switching circuit (52).
Given the aforementioned line interactive UPS, the main controller (53) determines the status of the current AC power supply and the power capacity of the rechargeable battery (55), and outputs the charge command or the discharge command to the charge and discharge mode controller (54). Furthermore, the charge and discharge mode controller (54) decides to execute a charging mode or a discharging mode. When the AC power supply normally supplies power, the main controller (53) controls the power switch set (52) to switch on or off so that the AC power is directly inputted to the load and the low-frequency transformer (51). If determining that the power capacity of the rechargeable battery (55) drops during this period, the main controller (53) controls the full-bridge switching circuit (52) through the charge and discharge mode controller (54) to convert the AC power into a DC current and charge the rechargeable battery (55) until the power capacity of the rechargeable battery (55) is full. However, once an irregularity or outage of the AC power is detected, the main controller (53) immediately switches off the power switch set (50) and outputs a discharge command to the charge and discharge mode controller (54). The charge and discharge mode controller (54) then activates the full-bridge switching circuit (52) to convert the DC power stored in the rechargeable battery (55) into an AC power and supply the AC power to the load, so as to realize interruptible power. As the UPS is always connected with the load, it is called a line interactive UPS. Although such UPS secures the rechargeable battery to maintain its full power capacity, charging to the rechargeable battery is performed through the conversion of the low-frequency transformer (51) and the full-bridge switching circuit (52). As a result, maintaining full power capacity of the rechargeable battery consumes more power.